The goal of this proposal is to understand synaptic transmission at the vestibular type I hair cell/calyx synapse. Three classes of morphological afferents have been described in the amniote crista and utricle. Calyx units contact type I hair cells (HCI) exclusively, bouton units contact type II hair cells only &dimorphic units receive innervation from both bouton &calyx fibers (Goldberg, 2000). The physiological response dynamics of these three classes of fibers vary, with calyx units having the most irregular firing pattern &the lowest gains to rotational stimuli (Baird et al. 1988;Lysakowski et al. 1995). The reasons for these variations are unclear, but are hypothesized to include differences in hair cell mechano-electrical transduction (MET) properties &differences in the biophysical membrane properties of primary vestibular afferents. We will study HCI &associated calyx afferents to determine how firing patterns in this unique terminal are shaped by both pre- &post-synaptic mechanisms. In Aim 1, patch clamp techniques will be used to study ionic conductances &transmitter release in a newly developed preparation of calyx terminals isolated together with HCI from gerbil vestibular organs (Rennie &Streeter, 2006). Excitatory postsynaptic currents (EPSCs) resulting from hair cell transmitter release will be recorded from calyx terminals under a variety of conditions. In Aim 2 we will record MET currents &receptor potentials from HCI during displacement of the hair bundle with a stiff probe in a wholemount utricle preparation. In Aim 3, mathematical modeling techniques will be employed to simulate HCI &calyx responses to glutamate. The passive electrical properties of the calyx &attached axon will be simulated with a segmented computational model in the NEURON programming environment. Na+, Ca2+ and K+ channels will be modeled with Hodgkin-Huxley style rate constants using the experimental data obtained in Aims 1&2. A genetic algorithm wiil be used to optimize the kinetic parameters for the activation &inactivation of ionic conductances. Dizziness is one of the most common medical complaints. Understanding the basic cellular mechanisms of balance sensation is essential to lay the groundwork for identifying causes &cures for this debilitating condition. The combination of experimental &modeling approaches will elucidate how sensory information is transformed by HC1 &converted into a neural code by their afferents.